One known method of electrical well logging is described in the U.S. Pat. No. 3,772,589, which issued to Scholberg. As described in this patent, a resistivity investigating sonde 10 such as shown in FIG. 1 of this application is suspended from a cable 12 in a borehole 14. The cable 12 is surrounded by a conductive sheath 16 such as an externally exposed armor, which, for a distance starting at the top of the sonde, is surrounded by an insulating sleeve 18 to form what is generally known as a bridle.
The sonde 10 generates, with a down-hole control 20, two types of survey currents at different frequencies of 280 Hz and 35 Hz. The high frequency survey current is intended to enable, by using one array of electrodes, the measurement of resistivity of the nearby formation while the lower frequency survey current enables, by use of another electrode array, the measurement of the resistivity of deeper formation regions. The sonde is provided with a plurality of electrodes, the central electrode, A.sub.o, of which introduces a survey current I.sub.o into the formation. Other electrodes known as guard electrodes A.sub.1, A.sub.2 are provided to generate bucking currents which aid to obtain a laterally deep penetration into the formation of the low frequency survey current I.sub.o before its upward redirection to a surface located return electrode (known as the B electrode). The B return electrode provides a return of both the survey and bucking currents.
The driving voltage V.sub.o for the survey current is sensed by measuring the potential between one of the sonde's voltage sensing electrodes, M, and a reference electrode N. This reference electrode N may be placed at a location 19 and near or connected to the bottom end 22 of the conductive sheath or armor 16 at the top of bridle 18 or be an isolated electrode such as N' at 19' located on the bridle 18 generally at some large distance from the upper end of sonde 10.
With the known sonde, resistivities are measured with a driver and analyzer network 24, commonly located on the surface and to which downhole sensed signals such as the sonde potential V.sub.o and survey current I.sub.o are transmitted via a telemetry cable link. A drive current such as I.sub.t is delivered by cable 12 to a downhole located control 20 to provide survey and bucking currents.
In the use of sonde 10, formation resistivities can be measured over a wide range of field conditions. Field conditions may be encountered, however, whereby, for example, the potential of the reference electrode N is influenced by an irregular return of currents to the surface electrode B and thus an error in the resulting resistivity measurement occurs. One of such field conditions has been found to occur when the current electrode B is located on the top of the bridle 18 above the reference electrode N, and when the sonde 10 is moved to a region immediately below a highly resistive bed 30 overlying a low resistive formation layer 32. In FIG. 1, this would correspond to B at 19 and N at 19'. The resulting resistivity measurement was found to be in error by a substantial factor. The solution to this problem involved a relocation of the current electrode B to the surface as shown in FIG. 1.
Notwithstanding the relocation of the current electrode B, anomalies in the measurement of resistivities with a sonde such as 10 at bed boundaries as depicted in FIG. 1 have been observed when operating with survey currents at a frequency of 35 Hz. The anomalies are normally characterized by a resistivity measurement at a value which leads one to believe a presence of hydrocarbons when in fact only salt water is found. Such misleading resistivity measurement leads to an undesirably expensive and unnecessary testing of the well.